Nickel phosphate nanorod-enhanced polyethylene oxide-based composite polymer electrolytes for solid-state lithium batteries

Journal Pre-proofs Nickel phosphate nanorod-enhanced polyethylene oxide-based composite polymer electrolytes for solid-state lithium batteries Zhijun Wu, Zhengkun Xie, Akihiro Yoshida, Jing Wang, Tao Yu, Zhongde Wang, Xiaogang Hao, Abuliti Abudula, Guoqing Guan PII: DOI: Reference:

S0021-9797(20)30005-9 https://doi.org/10.1016/j.jcis.2020.01.005 YJCIS 25878

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

26 October 2019 22 December 2019 3 January 2020

Please cite this article as: Z. Wu, Z. Xie, A. Yoshida, J. Wang, T. Yu, Z. Wang, X. Hao, A. Abudula, G. Guan, Nickel phosphate nanorod-enhanced polyethylene oxide-based composite polymer electrolytes for solid-state lithium batteries, Journal of Colloid and Interface Science (2020), doi: https://doi.org/10.1016/j.jcis.2020.01.005

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Nickel phosphate nanorod-enhanced polyethylene oxide-based composite polymer electrolytes for solid-state lithium batteries Zhijun Wua, Zhengkun Xiea, Akihiro Yoshidaa,b, Jing Wanga, Tao Yua, Zhongde Wangc, Xiaogang Haoc, Abuliti Abudulaa, Guoqing Guana,b,* aGraduate

School of Science and Technology, Hirosaki University, 1-Bunkyocho, Hirosaki 036-

8560, Japan. bEnergy

Conversion Engineering Laboratory, Institute of Regional Innovation (IRI), Hirosaki

University, 2-1-3, Matsubara, Aomori 030-0813, Japan. cDepartment

of Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024,

China.

Corresponding Author E-mail: [email protected]

1

ABSTRACT

Solid-state electrolytes with high ionic conductivity, large electrochemical window, and excellent stability with lithium electrode are needed for high-energy solid-state lithium batteries. In this work, a novel polyethylene oxide (PEO)-Lithium bis(trifluoromethylsulphonyl)imide (LiTFSI)-nanocomposite-based polymer electrolyte was prepared by using nickel phosphate (VSB-5) nanorods as the filler. The ionic conductivity of the obtained PEO-LiTFSI-3%VSB-5 solid polymer electrolyte was found to be as high as 4.8310-5 S·cm-1 at 30 C and electrochemically stable up to about 4.13 V versus Li/Li+. The enhanced ionic conductivity was attributed to the reduced crystallinity of the PEO and the interaction between VSB-5 and PEOLiTFSI. In addition, the solid polymer electrolyte exhibited improved compatibility to the lithium metal anode with excellent suppression of lithium dendrites. The assembled LiFePO4/Li battery with the PEO-LiTFSI-3%VSB-5 solid polymer electrolyte showed better rate performance and higher cyclic stability than the PEO-LiTFSI electrolyte. It is demonstrated that this new solid polymer hybrid should be a promising electrolyte applied in solid state batteries with lithium metal electrode. KEYWORDS: solid state battery; polymer electrolyte; nickel phosphate; high ionic conductivity; interface stability

1. Introduction Traditional lithium batteries (LIBs) with liquid electrolytes always have high energy density, good cyclability, and reliability, which have been an excellent choice for energy storage [1-3]. However, liquid electrolytes usually consist of organic solvents, which are flammable and

2

unstable at a high voltage, resulting in severe accidents [4-6]. Recently, solid-state batteries (SSBs) with solid-state electrolytes have attracted much attention due to their high energy density as well as high safety performance [7-13]. To date, various inorganic solid electrolytes have been developed for SSBs [14-17]. However, the applications of these inorganic solid electrolytes are still in a limited state owing to the lack of stiffness and film processability as well as high interface resistance with electrodes [18-21]. In contrast, solid polymer electrolytes (SPEs) are known to have relatively reliable stability, low interfacial resistance, flexibility, stretchable property, and low fabricating cost [22-26]. However, the low ionic conductivity (<10−6 S∙cm−1) of SPEs at ambient temperature and high-temperature instability restricts their wide utilization [27-29]. Among the developed SPEs, poly(ethylene oxide) (PEO)-based composite SPE is the most attractive one [30, 31]. In particular, the ionic conductivity can be tuned by changing the molar ratio of PEO to lithium salts and the preparation method [31]. To enhance the ionic conductivity and stability of SPEs, various attempts such as doping other materials, crosslinking, and block copolymerization have been tried [32-37]. It is reported that the addition of inorganic nanomaterials such as Al2O3, ZnAl2O4, BaTiO3, TiO2, SiO2, and montmorillonite (MMT) to the PEO-based SPEs can effectively enhance the ionic conductivity [38-42]. Moreover, sulfide and oxide electrolytes can be used as additive to enhance the performance of SPEs. For examples, the PEO18-LiTFSI-1%LGPS SPE containing 1% Li10GeP2S12 (LGPS) exhibited high ionic conductivities of 1.21×10−3 S∙cm−1 at 80 °C and 1.18×10−5 S∙cm−1 at 25 °C [32], and the ionic conductivity of PEO-LiClO4-LLZO SPE with 52.5% of Li7La3Zr2O12 (LLZO) reached 4.42×10−4 S∙cm−1 at 55 °C [43]. Furthermore, the incorporation of some fillers could inhibit the crystallization of PEO and weaken the interactions

3

between the lithium ions and the PEO chains for the further improvement of ionic conductivity and the stability of SPEs [6, 27, 44]. Nickel

phosphate

VSB-5

(Versailles-Santa

Barbara-5,

Ni20[(OH)12(H2O)6]

[(HPO4)8(PO4)4]∙12H2O), which was firstly synthesized by Ferrey and Cheetham et al.) is a nanoporous material with 24-ring open framework and high thermal stability [45]. It is found that VSB-5 has 1-D channels with a large pore size of 1.1 nm, which could provide the pathway for large molecules and lithium ions [46]. Considering these characteristics, herein, VSB-5 was selected as the filler for PEO-based SPE, and a series of PEO-LiTFSI-x%VSB-5 (x = 0, 1, 3, 5, and 10) SPEs were fabricated for the first time using a grinding process followed with a heat treatment way. The effect of VSB-5 addition amount on the crystallinity as well as the ionic conductivity was investigated via X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) analysis, scanning electron microscope (SEM) observation, electrochemical impedance spectroscopy (EIS) measurement, and differential scanning calorimetry (DSC) test. Meanwhile, the electrochemical windows of the prepared SPEs were evaluated by linear sweep voltammetry (LSV). Li+ transference number and the stability with lithium metal were tested by the Li symmetrical battery. The thermal stability of SPEs was also investigated by thermogravimetric analysis (TGA). Furthermore, the SSBs with the PEO-LiTFSI-x%VSB-5 SPE, LiFePO4-based cathode, and Li anode were assembled and their electrochemical performances were evaluated. 2 Experimental Section 2.1 Synthesis of VSB-5 nanorods: VSB-5 nanorods were synthesized by hydrothermal synthesis method. Briefly, 1 mmol of nickel chloride (NiCl2∙6H2O) and 10 mmol of hexamethylenetetramine (HMT) were dissolved in

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25 mL of water at first. It is known that the properties such as particle size and crystallinity are different depending on the synthetic conditions [47]. To obtain a variety of VSB-5 nanorods, varying amounts of dibasic sodium phosphate (NaH2PO4) were added under stirring and meanwhile the pH of the solution was adjusted to 7 or 9 by adding NH3∙H2O or HCl. After stirring for 30 min, the final gel was kept in a Telfon-lined stainless autoclave and heated in an oven at 140 °C for 8 h. The powder products were washed repeatedly with distilled water, filtered, and dried at 100 °C. Finally, the as-synthesized samples were calcined at 350 °C in air for 1 h. 2.2 Fabrications of the cathode and solid polymer electrolyte PEO (MW = 6×105), LiTFSI, and VSB-5 were completely dried before use. PEO, LiTFSI, and VSB-5 were mixed using an agate mortar to obtain a rough, soft cloth state material. Herein, the amount of LiTFSI was determined by the molar ratio of (EO)/Li+ (EO/Li+=8), and the quantity of VSB-5 was decided by the mass ratio of VSB-5/LiTFSI-PEO (0, 1, 3, 5, 10 wt%). The obtained rough sheet film was sealed in the glove box and transferred to a vacuum oven at 80 °C and dried for 12 h. Subsequently, it was pressed under 5 MPa for 2 h, and as such, a uniform composite membrane with a thickness of 80-120 µm was obtained. For the fabrication of cathode, LiFePO4 particles, conductive additive super P and polyvinylidene fluoride (PVDF) with a weight ratio of LiFePO4:super P:PVDF=8:1:1 were dispersed in a certain amount of N-methyl-2-pyrrolidone (NMP) solvent and stirred for 12 h to get a homogenous slurry, and then the slurry was coated on an Al current collector and dried at 120 °C in vacuum for 12 h. 2.3 Characterizations

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Differential Scanning Calorimetry (DSC) measurement of the SPE was performed by using a thermal analyzer (SII, DSC 6200, Japan) with a heating rate of 10 °C∙min-1. X-ray diffraction (XRD) pattern was collected in a 2θ range from 10° to 90° on an X-ray diffractometer (Smartlab 9kW, Rigaku, Japan) with the Cu Kα radiation (λ = 1.5406 Å). The morphology was characterized using a scanning electron microscope (SEM) (SU8010, Hitachi, Japan). Thermogravimetric analysis (TGA) was conducted on a TA machine (TA-60WS, SHIMADZU, Japan) to investigate the thermal stability of the SPE. Chemical structures of PEO, LiTFSI, VSB5, PEO-LiTFSI, PEO-LiTFSI-VSB-5 were determined by the Fourier transform infrared spectrum (FT-IR) (FT/IR-4200, JASCO, Japan). 2.4 Electrochemical evaluation The obtained SPE film was sandwiched by stainless steel foils (SS) to form a SS/SPE/SS battery for the ionic conductivity measurement by using the electrochemical impedance spectroscopy (EIS) (VersaSTAT 4, Princeton Applied Research, USA), in which the temperature was set in a range of 30-100 C with a frequency range of 10-106 Hz. For measuring Li+ transference number (

) of SPE, the Li/SPE/Li battery with Li foils was assembled, and its DC

polarization property was investigated. Herein, a voltage of 10 mV was applied on the battery, and EIS spectra of battery before and after polarization were obtained in a frequency range of 102-106

=

Hz. The

is calculated based on the following equation: Eq. (1)

where ∆V is the applied DC polarization voltage (0.01 V), I0 and Is are initial and stable currents during the polarization, R0 and Rs are the resistances of the SPE before and after the polarization. Linear sweep voltammetry (LSV) curves were recorded on the electrochemical workstation in a

6

voltage range of 0-5.5 V by using Li/SPE/SS 2025 cells. The stability of the interface between SPE and Li electrode was examined with Li symmetrical battery by a stripping/plating test at 0.01 mA. The solid batteries were assembled in 2025-type coin cells with lithium metal anode, PEOLiTFSI-VSB-5 composite electrolyte and LiFePO4 cathode. Galvanostatic charge and discharge performances of the solid-state batteries were investigated using a LANHE CT2001A charge/discharge system (Wuhan LAND Electronics Co., Ltd.) in a potential range of 4.0-2.5 V at 60 °C. 3 Results and discussion In order to prepare VSB-5 with suitable morphology, the effect of initial pH as well as NiCl2∙6H2O/HMT/NaH2PO4 ratio on the morphology of VSB-5 was investigated. As shown in Fig. 1, the sizes of VSB-5 nanorods were large and inhomogenous with about 270-320 nm in diameter

and

several

micrometers

in

length

as

it

was

prepared

with

a

NiCl2∙6H2O:HMT:NaH2PO4 molar ratio of 1:10:5 at pH = 7 (Fig. 1A). Meanwhile, when the pH was increased to 9, a mixture of flower-like particles was obtained (Fig. 1B). However, when the NiCl2∙6H2O/HMT/NaH2PO4 ratio was changed from 1:10:5 to 1:10:15 (Figs. 1C-D), the rods became thinner (as compared to Fig. 1A), and at pH = 7, the VSB-5 nanorods had a diameter in the range of 180-220 nm with different lengths. In contrast, as pH = 9, the VSB-5 nanorods became thinner with a uniform size, which had a diameter of approximately 100 nm, and a length of 3 um (Fig. 1D). Herein, the flower-like VSB-5 particles were only observed from the SEM image as the molar ratio of NiCl2∙6H2O:HMT:NaH2PO4 = 1:10:5 at pH = 9. In general, the aligned nanorods could provide channels for the fast ion transport owing to no crossing junctions

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[6, 27, 44]. Therefore, the VSB-5 nanorods prepared with the NiCl2∙6H2O:HMT:NaH2PO4 molar ratio of 1:10:15 at pH = 9 was selected for further studies in this work.

Fig. 1 SEM images of VSB-5 (A) pH=7 (MR= 1:10:5) (B) pH=9 (MR= 1:10:5) (C) pH=7 (MR= 1:10:15) (D) pH=9 (MR= 1:10:15), MR: molar ratio of NiCl2∙6H2O:HMT:NaH2PO4. The electrochemical impedance spectroscopies of SPEs with different VSB-5 addition amounts, i.e., PEO-LiTFSI-x%VSB-5 (x = 0, 1, 3, 5, and 10) using the SS/SPE/SS batteries were measured in a frequency range of 10–106 Hz at a temperature range 30-100 °C. As shown in Fig. 2, a semicircle and a spike were observed in the high- and low-frequency ranges, respectively, for each battery. The semicircle represents the bulk resistance of SPE and the interfacial resistance between the SPE and SS electrode, and the spike is resulted from the ion diffusion

8

impedance in the electrode. As such, the ionic conductivity value (σ) can be calculated from the intercept of the spike using the following equation: σ=

Eq. (2)

where σ, d, R, and S are the ionic conductivity, the thickness, the volume impedance, and the surface area of the SPE, respectively. From Fig. 2, one can see that the ionic conductivity increased after VSB-5 was introduced into the PEO-LiTFSI SPE. The calculation results based on eq. (2) indicated that the PEO-LiTFSI-3%VSB-5 had the highest ionic conductivity of 4.8310-5 S·cm-1 at 30 C, which is approximately three times higher than that (1.5210-5 S·cm-1 at 30 C) of the PEO-LiTFSI SPE. With further increase in VSB-5 addition amount, the ionic conductivity of PEO-LiTFSI-x%VSB-5 began to decrease. Herein, the excessive VSB-5 nanorods filling in polymer matrix could lead to the aggregation and free-volume depletion of the nanorods, which would cause a decrease of ionic conductivity [6, 44, 48]. Therefore, the PEO-LiTFSI-3%VSB-5 SPE was selected as the optimized SPE for further study.

9

7000

PEO-LiTFSI d = 0.142 mm PEO-LiTFSI-1%VSB-5 d = 0.06 mm PEO-LiTFSI-3%VSB-5 d = 0.084mm PEO-LiTFSI-5%VSB-5 d = 0.155 mm PEO-LiTFSI-10%VSB-5 d = 0.128 mm

6000

-Z'' ()

5000 4000 3000 2000 1000 0

0

500

1000

1500

2000

2500

Z' () Fig. 2 EISs of PEO-LiTFSI-x%VSB-5 (x = 0, 1, 3, 5, and 10) at 30 C. d: Thickness. Fig. 3A shows XRD patterns of the PEO-LiTFSI-x%VSB-5 (x = 0, 1, 3, 5, and 10) SPEs. The regions of pure PEO appearing between 2θ = 15.1 to 30.2 exhibited the amorphous phase of PEO whereas the peaks at 2θ = 19.6, 23.8, 26.7, and 27.4 corresponded to the crystalline phase of PEO. The simultaneous presences of halo region and crystalline peaks indicate the semi crystalline nature of PEO [49, 50]. Two apparent different diffraction peaks at 11.1 and 14.6 were observed for the PEO-LiTFSI electrolyte [51], which caused local depletion of the lithium ions in the PEO amorphous phase. As VSB-5 was added into PEO-LiTFSI electrolyte, the diffraction peaks of VSB-5 became very weak. Fig. 3B shows the high-resolution XRD pattern of PEO-LiTFSI-3%VSB-5 SPE. Weak diffraction peaks corresponding to VSB-5 were observed. As shown in Fig. 3A, the intensity of the PEO peaks was decreased, and the halo region of PEOLiTFSI-x%VSB-5 was broadened with the increase of x (x = 1, 3, 5, and 10), indicating the

10

decrease of PEO crystallinity and the increase of PEO amorphous phase, which could enhance the ion conductivity of such SPEs.

Fig. 3 XRD (A) of the PEO-LiTFSI-x%VSB-5 (x = 0, 1, 3, 5, and 10) and high-resolution XRD (B) of VSB-5 and PEO-LiTFSI-3%VSB-5. Fig. 4A illustrates the interactions between different components including PEO, LiTFSI, and VSB-5. Herein, the VSB-5 nanorods could interact with TFSI− anion in the lithium salt and weaken the interaction between Li+ and TFSI− so that Li+ movement in the electrolyte becomes more easily. Meanwhile, the VSB-5 nanorods could interact with PEO to alter the polymer arrangement and/or crystallinity. In addition, the nanostructure of VSB-5 could provide rich Li+ transport channels. All of these could increase the whole ionic conductivity. Fig. 4B shows the FTIR spectra of PEO, VSB-5, LiTFSI, and PEO-LiTFSI-x%VSB-5 (x = 0, 1, 3, 5, and 10) in the wavenumber range from 4000 to 400 cm-1. Herein, the peaks at 1203 and 797 cm−1 are assigned to the -CF3 bonds of LiTFSI [52] and the characteristic peaks at 2887 cm-1, 1467 cm-1, 1343 cm1,

1244 cm-1, 964 cm-1, and 839 cm−1 belong to PEO. The band observed at the 839 cm-1 is

associated with the CH2 rocking mode and/or the C-O stretching mode; the band located at the 964 cm-1 is attributed to the symm-asymm C-O-C stretching mode; and the band observed at the

11

1244 cm-1, 1343 cm-1 and 1467 cm-1 corresponds to the asymmetric CH2 twisting, CH2 bending/wagging, and C-H bending respectively [53]. Fig. 4C shows the magnification spectra in the range of 1500-900 cm-1. The peaks centered at 1146 cm-1, 1329 cm-1, and 957 cm-1 are attributed to asymmetric -SO2- stretching in the PEO-LiTFSI electrolyte. However, these peaks shifted to 1140 cm−1, 1331 cm−1, and 952 cm−1 respectively with the addition of VSB-5, indicating the interaction between VSB-5 nanorod and the -SO2- group in the TFSI- [6]. In addition, one can see that the addition of the VSB-5 also reduced the intensity of peaks related to the PEO. Herein, VSB-5 has active P–OH groups on the channel surface [54], and PEO has C-O groups based on the FTIR analysis result. As such, VSB-5 could be connected with PEO via HOH hydrogen bonds. The intermolecular hydrogen bonding effect between PEO and VSB-5 enables polymer chains to be more disordered in the SPE and change the polymer arrangement and/or decreased its crystallinity. The intrinsically increased disorder and amorphous content of the polymer chains could also enhance the Li+ transmission by releasing more Li cations and further improved ionic conductivity [6]. These are in agreement with the XRD results. Differential scanning calorimetry (DSC) was carried out to analyze the thermal behavior of PEO-LiTFSI-x%VSB-5 (x = 0, 1, 3, 5, and 10) electrolytes. As shown in Fig. 5A, the melting transition of PEO-LiTFSI was near 53.6 °C. With the addition of VSB-5, the melting transition of PEO-LiTFSI-3%VSB-5 electrolyte decreased to nearly 49.2 °C while the peak area also decreased significantly. Generally, the lower melting point and peak area mean a lower degree of crystallinity [55]. Thus, it confirmed that the addition VSB-5 in the SPE effectively decreased the crystallinity of PEO and increased PEO segmental mobility, which should be benefit for the improvement of ionic conductivity as stated above. This is also in agreement well with the XRD results.

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Fig. 4 Schematic (A) of Li+ migration in VSB-5 enhanced composite SPEs, FTIR (B) of the PEO-LiTFSI-x%VSB-5 (x = 0, 1, 3, 5, and 10) and the magnification (C) of FTIR in the wavenumber range of 1500-900 cm-1.

Fig. 5 DSC (A) of the PEO, PEO-LiTFSI and PEO-LiTFSI-3%VSB-5 and the EIS and chronoamperometry curves (B) of a Li/SPE/Li cell under a potential step of 10 mV.

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To confirm the effect of VSB-5 addition on the migration of Li+ in PEO-LiTFSI electrolyte, the lithium ion transference number (

) of PEO-LiTFSI-3%VSB-5 electrolyte was also

calculated by Eq. (1) based on the measurement results of chronoamperometry (Fig. 5B). As a result, the

of PEO-LiTFSI-3%VSB-5 electrolyte was 0.13, indicating that the

can be

effectively improved by the addition of VSB-5. The electrochemical operation windows of the fabricated SPEs were examined by the LSVs of the SS/SPE/Li batteries. As shown in Fig. 6A, the electrochemical window of PEO-LiTFSI3%VSB-5 electrolyte was nearly 4.08 V, which was higher than that of the PEO-LiTFSI electrolyte (4.13 V), suggesting that the addition of VSB-5 effectively protected the PEO matrix from side reactions [56]. Meanwhile, the thermal stability of PEO-LiTFSI-3%VSB-5 electrolyte was also tested by TGA. As shown in Fig. S1, the obvious weight loss was observed at a temperature range of around 340-390 °C in the case of PEO-LiTFSI-3%VSB-5 electrolyte, which should be mainly attributed to the partly decomposition of PEO and the loss of bound water in the VSB-5, and the continuous weight loss occurred in the temperature range of 360-590 °C should be resulted from the complete decomposition of PEO and LiTFSI [6]. Finally, the residual weights of PEO-LiTFSI-3%VSB-5 and PEO-LiTFSI electrolytes after 660 °C were about 7.0 wt% and 4.8 wt%, respectively. Besides electrochemical and thermal stability, the interface stability of PEO-LiTFSI3%VSB-5 electrolyte with the Li foil electrode was also analyzed by galvanostatic cycling test of the Li/SPE/Li batteries with a constant current of 0.1 mA at 60 °C. As shown in Fig. 6B, the Li plating and stripping over-potential of the PEO-LiTFSI-3%VSB-5 electrolyte was near 40 mV and maintained stable for 480 h, indicating good compatibility between PEO-LiTFSI-3%VSB-5 electrolyte and Li electrode. In contrast, the battery with the PEO-LiTFSI electrolyte had a

14

higher overpotential of 80 mV at the first 70 h, and suddenly the battery got short-circuited. Thus, the addition of VSB-5 in the PEO-LiTFSI electrolyte resulted in good compatibility with Li metal and exhibited a positive effect on inhibiting the uncontrollable formation and growth of Li dendrite.

Fig. 6 LSVs (A) of the PEO-LiTFSI and PEO-LiTFSI-3%VSB-5 SPEs under a scan rate of 1 mV∙s−1 and voltage-time profiles (B) of Li metal plating and stripping in Li/SPE/Li cells at 0.1 mA. The microstructure and elemental distribution were also characterized by SEM and EDX mapping (F, S, P and Ni). As shown in Fig. S2A, smooth surface morphology of PEO-LiTFSI3%VSB-5 SPE was observed, indicating the satisfactory miscibility between the PEO-LiTFSI and VSB-5. Meanwhile, as shown in Fig. S2B, the distributions of P and Ni almost overlapped with the signals of F and S, indicating that VSB-5 was homogeneously dispersed in the PEOLiTFSI-3%VSB-5 electrolyte. As shown in Fig. S3, PEO and VSB-5 were clearly observed, in which the PEO had a block-like structure and VSB-5 was still in the nanorod-like state without any changes. Moreover, Fig. S4 shows TEM images of the PEO and PEO-VSB-5. It is obvious that the PEO was composed of nanoparticles (Fig. S4A and B) and VSB-5 nanorods combined

15

well with PEO in the PEO-VSB-5, also indicating the good compatibility between PEO and VSB-5.

Fig. 7. (A) Cycling performances of LiFePO4/PEO-LiTFSI/Li and LiFePO4/PEO-LiTFSI3%VSB-5/Li batteries at 0.2 C and 60 °C. (C) Rate performances of LiFePO4/PEO-LiTFSI/Li and LiFePO4/PEO-LiTFSI-3%VSB-5/Li batteries at 60 °C. To evaluate the cycle stability, the batteries were tested in the charge/discharge cycling with a cut-off potential range of 2.5 to 4 V (vs Li/Li+). The cycling performance of LiFePO4/SPE/Li at 0.2 C and 60 °C is presented in Fig. 7A. One can see that the discharge capacity decreased with the increase in the discharge-charge cycle number. The discharge capacity of the LiFePO4/PEOLiTFSI/Li battery decreased to 126.6 mAh∙g-1 after the 50-cycle test. In contrast, the discharge capacity of the LiFePO4/PEO-LiTFSI-3%VSB-5/Li battery was still 157.4 mAh∙g-1 after the 50cycle test. In addition, the coulombic efficiency of LiFePO4/PEO-LiTFSI-3%VSB-5/Li battery was still stable at more than 99% whereas that of LiFePO4/PEO-LiTFSI/Li battery had fluctuated and decreased after 43 cycles. Fig. 7B shows the rate performances of the SSBs based on PEOLiTFSI and PEO-LiTFSI-3%VSB-5 SPEs and Li anode. The LiFePO4/PEO-LiTFSI-3%VSB5/Li battery delivered the discharge capacities of 140.4, 139.4, 134.9, 127.8 and 136.4 mAh∙g-1 at rates of 0.1, 0.3, 0.5, 1 and 0.1 C, respectively. When the rate was returned to 0.1 C, the

16

discharge capacity maintained as high as 97.2% of the initial value. In contrast, the discharge capacities of LiFePO4/PEO-LiTFSI/Li battery were 111.8, 124.9, 119.1, 98.2 and 130.1 mAh∙g-1 at 0.1, 0.3, 0.5, 1 and 0.1 C, respectively. The PEO-LiTFSI-3%VSB-5 SPEs could improve the capacity of solid battery. Thus, the addition of VSB-5 enhanced the cycle stability as well as capacity maintaining ability due to the improved Li+ conductivity, higher stability with lithium, and the lower interface resistance between the electrode and SPE. The interface images between the SPE and the Li electrode were observed by SEM. Figs. 8A and 8C show the sandwich structures of the two LiFePO4/PEO-LiTFSI/Li and LiFePO4/PEOLiTFSI-3%VSB-5/Li batteries, in which the thicknesses of the SPEs were in the range of 30-50 μm. However, as shown in Fig. 8B, after the cycling test at 60 °C, in the case using the PEOLiTFSI SPE, the Li anode exhibited an uneven surface with massive Li dendrites. In contrast, a relatively flat surface was observed on the Li anode when PEO-LiTFSI-3%VSB-5 SPE was used (Fig. 8D). It demonstrated that the PEO-LiTFSI-3%VSB-5 SPE should be more favorable to inhibit the growth of Li dendrites. Thus, the addition of VSB-5 nanorods can effectively suppress Li dendrite formation.

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Fig. 8 Cross-section and interface images of LiFePO4/PEO-LiTFSI/Li battery (A-B) and LiFePO4/PEO-LiTFSI-3%VSB-5/Li battery(C-D) after the rate cycling test. 4. Conclusions In summary, VSB-5 nanorods with a thin and uniform size were successfully prepared by the hydrothermal synthesis method and used as the filler in the PEO-LiTFSI SPE for SSBs. The addition of VSB-5 in PEO-LiTFSI SPE effectively improved the thermal stability, lithium stability, ionic conductivity, interfacial stability, and operation electrochemical window. As a result, the ionic conductivity of PEO-LiTFSI-3%VSB-5 reached 4.8310-5 at 30 °C with a widened electrochemical stability window up to 4.13 V vs. Li/Li+. It is found that the high ionic conductivity of PEO-LiTFSI-3%VSB-5 was resulted from not only the interaction between the VSB-5 nanorod and -SO2- in the TFSI-, which could promote the transport ability of Li+, but also

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the interaction between the VSB-5 nanorod and the PEO, which significantly reduced the glass transition temperature and degree of crystallinity of PEO-LiTFSI-3%VSB-5 SPE so that the ionic conductivity was also enhanced. Furthermore, the LiFePO4/PEO-LiTFSI-3%VSB-5/Li battery exhibited the improved cyclic performance as well as the rate capability compared with the LiFePO4/PEO-LiTFSI/Li battery due to the suppression effect of lithium dendrites and high ionic conductivity of PEO-LiTFSI-3%VSB-5 SPE. It is demonstrated that this new solid polymer composite electrolyte should be a promising electrolyte applied in the solid state batteries with lithium metal electrode. Acknowledgments This work is supported by Jiku Chemical Co. Ltd. Z. Wu and Z. Xie gratefully acknowledge China Scholarship Council. References [1] J. Lu, Z. Chen, Z. Ma, F. Pan, L.A. Curtiss, K. Amine, The role of nanotechnology in the development of battery materials for electric vehicles, Nat. Nanotechnol. 11 (2016) 1031. [2] Y. Lu, Z. Tu, L.A. Archer, Stable lithium electrodeposition in liquid and nanoporous solid electrolytes, Nat. Mater. 13 (2014) 961. [3] Y. Tao, G. Zeng, C. Xiao, Y. Liu, Y. Qian, J. Feng, Porosity controlled synthesis of nanoporous silicon by chemical dealloying as anode for high energy lithium-ion batteries, J. Colloid Interface Sci. 554 (2019) 674-681. [4] B. Scrosati, J. Garche, Lithium batteries: status, prospects and future, J. Power Sources 195 (2010) 2419-2430.

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Credit author statements:

Guoqing Guan: Conceptualization; Methodology; Validation; Writing - Review & Editing; Supervision; Project administration. Zhijun Wu: Experimental; Formal analysis; investigation; Data curation; Writing - Original Draft; Zhengkun Xie and Jing Wang: Experiment assistance and discussion. Akihiro Yoshida: Data discussion; Writing - Review & Editing (secondary checking) Tao Yu: Writing - Review & Editing (Third checking) Zhongde Wang and Xiaogang Hao: Data discussion and advice Abuliti Abudula: Conceptualization (advice), Project administration.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights



PEO-LiTFSI-3%VSB-5 electrolyte exhibits high conductivity of 4.8310-5 S·cm-1 at 30 C.



PEO-LiTFSI-3%VSB-5 electrolyte shows better interface stability with lithium.



The batteries with PEO-LiTFSI-3%VSB-5 exhibit excellent electrochemical property.



VSB-5 can change the polymer arrangement and decrease the crystallinity.

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